This invention relates generally to an electrostatographic printing machine, and more particularly concerns an improved development system for use therein.
The process of electrostatographic printing includes both electrographic printing and electrophotographic printing. In both of the foregoing processes an electrostatic charge pattern or latent image corresponding to the original document being reproduced is recorded on an image bearing member. Electrophotographic printing is disclosed in U.S. Pat. No. 2,297,691 issued to Carlson in 1942. As described therein, a photosensitive element having a photoconductive insulating layer is charged to a substantially uniform potential. The charged photoconductive surface is then exposed to a light image of the original document. The light image selectively dissipates the charge in the irradiated areas and creates an electrostatic latent image on the photoconductive surface. Electrographic printing creates an electrostatic latent image, without the employment of a photosensitive material or a light image.
While the foregoing processes form an electrostatic latent image in differing manners, the latent image is usually developed by bringing a developer mix into contact therewith. A typical developer mix generally comprises toner particles, such as colored thermoplastic particles, which electrostatically adhere to coarser carrier granules, such as ferromagnetic granules. The toner particles are usually heat settable.
Various types of developing systems are employed in the art. These systems include, amongst others, cascade development, magnetic brush development, powder cloud development and liquid development. Magnetic brush systems achieve a high degree of uniform deposition and, therefore, numerous electrostatographic printing machines employ this type of system.
Multi-color electrostatographic printing involves the utilization of various process components to produce a series of electrostatic latent images corresponding to a particular color in the original document. In such a system, there is a requirement to develop successive partial color images. Each color image is developed with toner particles of a selected color. These powder images are then transferred to a suitable support surface, in registration with one another, to form a reproduction of the colored original document.
In magnetic brush systems, frequently the local contrast in the developed image is extremely high. Development systems frequently produce a powder cloud or quantity of toner particles disposed in the region of the development zone. Heretofore, the foregoing powder cloud was treated as an undesirable effect and measures were generally taken to suppress the formation thereof. However, it has now been found that the hereinbefore unwanted powder cloud may be employed to suppress the effects of local contrast.
Accordingly, it is a primary object of the present invention to improve the apparatus employed in developing electrostatic latent images so as to suppress the effects of local contrast and produce a softer or less harsh copy.